1. Field of the Invention
The present invention relates in general to internal combustion engines of automotive vehicles and more particularly to a device for controlling the phase of a camshaft for operating a valve of an internal combustion engine and thereby controlling the valve timing in order to obtain optimum efficiency and performance of the engine throughout the operating speed.
2. Description of the Prior Art
A valve timing control device of the type adapted to control the phase of a camshaft (i.e. a camshaft angle relative to a crankshaft angle) by advancing and retarding a power transmitting member such as for example a timing belt used in a drive arrangement connected to the camshaft is disclosed, for example, in Japanese Provisional Patent Publication No. 52-31214 and also shown in FIG. 4.
In the figure, 41 is a crankshaft serving as an output shaft of an engine, and 42, 43 are camshafts mounted above a cylinder head (no numeral) to control the opening and closing of intake and exhaust valves (not shown), respectively. On the crankshaft 41 a driving wheel 42 is mounted and on the camshafts 42, 43 a pair of driven wheels 45, 46 are mounted, respectively. Indicated by the reference numeral 47 is a power transmitting member in the form of a single, endless strip, passing about the driving and driven wheels 44-46.
The power transmitting member 47 is in effect a timing chain, timing belt or flat belt. In the case of a timing chain or belt, sprockets are used as the driving and driven wheels 44-46 and in the case of a flat belt pulleys are used. Hereinbelow, the power transmitting member 47 is described as a timing belt but not limited to it.
The timing belt 47 have three belt sections extending between the adjacent two of the sprockets 44-46, which belt sections respectively pass over idler pulleys 48-50 disposed outside of the timing belt 47 to serves as adjusting wheels for adjusting the lengths of the timing belt sections and thereby varying the valve timing. The idler pulleys 48-50 are driven by a drive unit including cylinders 51-53 and movable toward and away from the timing belt 47. The cylinders 51-53 are extensible and contractible in response to variation in various engine parameters such as engine r.p.m., load, etc.
With the above described valve timing control device, upon adjustment of the relative phase between the camshafts 42, 43 through adjustment of the length of the timing belt section extending between the sprockets 45, 46, it is necessary to move the idler pulleys 48, 49 toward or away from the associated timing belt sections in order to hold them in a suitably tensioned state. In this connection, since the camshafts 42, 43 are spaced a relatively large distance from the crankshaft 41, the length of the timing belt section extending between the sprockets 44, 45 or 44, 46 is relatively large, resulting in the necessity that the maximum strokes of the idler pulleys 48, 49 are large. Due to this, the drive unit for the idlers 48, 49 inevitably becomes large-sized to increase the width of the engine, causing a difficulty in the arrangement of engine accessories such as a water pump pulley, etc.
Furthermore, due to the fact that the crankshaft 41 is spaced a relatively large distance from the camshafts 42, 43, the timing belt sections extending between the sprockets 44, 45 and 44, 46 are swingable largely during the time when the valve timing is being varied, resulting in the possibility that the timing belt skips over some of the teeth of the sprockets 44-46.
Still furthermore, since increase in the length of the timing belt section between the sprockets 44, 45 or 44, 46 due to secular change is large since the distance which the crankshaft 41 is spaced from the camshafts 42, 43 is large, predetermined movements of the idler pulleys 48, 49 become inappropriate, tending to make the valve timing inappropriate to cause deterioration in the engine output, fuel consumption, exhaust gas components, etc.
Yet furthermore, since the camshafts 42, 43 are adapted to rotate at half the speed of the crankshaft 41, the diameter of each sprockets 45, 46 is twice as large as that of the sprockets 44. For this reason, the distance between the camshafts 42, 43 cannot be reduced to a satisfactory extent, resulting in an increased height of the engine since relative inclination between the intake and exhaust valves cannot be made small to a satisfactory extent and further since the idler pulley 50 and its associated drive unit parts need to be disposed adjacent the outer periphery of the engine.